Seismologists are worried the recent earthquake swarm at the Salton Sea will temporarily increase the likelihood of a major quake in Southern California.

Southern Californians learn to live with the risk of earthquakes.

But over the last week, anxieties were particularly heightened, and the natural denial that is part of living in earthquake country was harder to pull off.

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A swarm of seismic activity at the Salton Sea that began a week ago prompted scientists to say there was an elevated risk for a big San Andreas fault earthquake. By Tuesday, that risk had lessened, and the California governor's Office of Emergency Service said that the earthquake advisory period was over.

But the impact of that warning was still being felt. For some, it meant checking quake safety lists. Others looked at preparing for the Big One, such as bolting bookshelves to walls, installing safety latches on kitchen cabinets and strapping down televisions.

San Bernardino, which is on the San Andreas Fault, took the unprecedented step of closing down City Hall through Tuesday over concerns about how the structure would fare in the big quake.

"We haven't had an alert like this," Mark Scott, San Bernardino's city manager, said. "We're not trying to suggest that the alert is an impending catastrophe. We're just trying to use an abundance of caution. We care about the safety of the public and our employees."

Salton Sea earthquake (Los Angeles Times)

The city had already been planning to vacate the seven-story building within the next few months.

The City Hall building was constructed before the Sylmar earthquake in 1971, according to Scott. After that earthquake, he said, seismic codes changed significantly.

"Other earthquakes have changed California seismic codes even further, to the point where the current City Hall building in San Bernardino is an example of everything you would not build today," Scott said.

A 2007 report noted the potential instability of the building in the event of a major earthquake, Scott said.

The estimated cost of retrofitting the building is more than $20 million, Scott said.

"For a bankrupt city, that's a tough challenge," Scott said. "We're looking at alternative approaches, rather than doing $20 million worth of work."

The risk of a big quake was highest during and directly after the quake swarm, experts said.

Thomas H. Jordan, director of the Southern California Earthquake Center, said Monday afternoon that the earthquake swarm about 150 miles southeast of Los Angeles has been "decaying away nicely. It's been tailing off.

"I would say the risk is declining," Jordan said. Of swarm activity at the Salton Sea, "it really hasn't been very active in the last couple of days. It's been pretty quiet."

U.S. Geological Survey research seismologist Rob Graves said he thought the elevated risk of a San Andreas earthquake has largely receded to the background level of risk found in any given week.

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"It's getting pretty close to the background level — which is not zero; there's always a chance of having a major earthquake," Graves said. "It's been almost a week since the swarm activity has subsided, and so that's about the time window we'd be looking at for its effects to diminish."

Seismologists have been closely tracking a swarm of earthquakes just south of the end of California's mighty San Andreas fault, which experts consider overdue for a major earthquake, with a magnitude of 7 or greater.

Any significant seismic activity near a major earthquake fault is a cause for concern, scientists say, because of the fear it could awaken it and trigger a massive temblor.

The San Andreas fault's southernmost stretch has not ruptured since about 1680 — more than 330 years ago, scientists estimate. And a big earthquake happens on average in this area once every 150 or 200 years, which is why the region is long overdue for a major quake.

Last Tuesday — Sept. 27 — the U.S. Geological Survey issued a statement that the chances of a magnitude 7 or greater earthquake being triggered on the southern San Andreas fault over the next seven days were as high as 1 in 100 and as low as 1 in 3,000.

Without the swarm, the average chance for such an earthquake striking on any given week is 1 in 6,000, according to Jordan.

That seven-day period ends Tuesday morning.

The swarm began just after 4 a.m. on Sept. 26, starting earthquakes three to seven miles deep underneath the Salton Sea.

The biggest earthquakes hit later that morning — a 4.3 and then a pair later at night, another 4.3 followed by a 4.1. There was another burst of activity the following night.

It marked only the third time since earthquake sensors had been installed in this area in 1932 that the area had seen such a swarm. And this swarm had more earthquakes than events in 2001 and 2009.

Earthquake experts say it's important that the elevated risk of a major event from a seismic swarm be clearly communicated to the public.

For San Bernardino, the quake swarm was just latest reminder of the seismic risks the city faces.

San Bernardino has one of the largest concentrations of unreinforced masonry buildings in the state that are at risk of particularly intense ground motion.

"I was shocked to hear that, because I've been in several other cities and a lot of cities [retrofitted unreinforced masonry buildings] 30 years ago," Scott said.